1. Field Of The Invention
The present invention relates to a source reagent composition and method for liquid delivery chemical vapor deposition of carbide films on substrates, in a specific aspect, the invention relates to a silicon carbide source reagent composition and liquid delivery chemical vapor deposition method for depositing silicon carbide coatings on substrates such as surfaces, fibers, composite matrices and semiconductor device structures.
As used herein, the term "carbide" is intended to be broadly construed, to encompass carbides per se, as well as oxycarbides, within its scope.
2. Description of the Related Art
Carbides are commonly used in surface and interstitial coating applications where specific physical and/or electronic characteristics are desired. In physical applications, carbide coatings provide superior protection against heat, abrasion and corrosion. Additionally, carbide-based materials are coated on fiber matrices to form high-strength ceramic matrix composites. Under appropriate process conditions, various carbide materials can also be made with desirable optical, electrical and semiconductive characteristics.
Fiber interface coatings can be applied to surfaces by using chemical vapor deposition (CVD) to achieve coatings on fibers or coatings over structures, or chemical vapor deposition in an infiltrative mode, viz., chemical vapor infiltration (CVI), to achieve matrices and fiber coatings within open structures. CVD is an attractive method for forming thin film coatings of various types, because CVD is readily scaled up to production runs, and the electronic and semiconductor industry has experience and an established equipment base in the use of CVD technology that can be applied to new CVD processes.
Whether forming carbide coatings on structures, fibers or matrices, or forming thin films for semiconductor or optical applications, a wide variety of source materials may be employed for CVD/CVI processes. These source materials include reagents and precursor materials of varying types, and in various physical states. To achieve highly uniform thickness layers of a conformal character on a semiconductor substrate, vapor phase deposition is used widely as a technique. Even in less controlled environments, for the coating of structures, fibers or matrices, CVD or CVI processes can be employed using varying reagents and precursor materials.
In vapor phase deposition, the source material may be of initially solid form which is sublimed or melted and vaporized to provide a desirable vapor phase source reagent. Alternatively, the reagent may be of normally liquid state, which is vaporized, or the reagent may be in the vapor phase in the first instance.
As used herein, the term "liquid delivery" when referred to chemical vapor deposition or other thin film or coating process refers to the fact that the precursor or source reagent composition for the material to be deposited on a substrate is vaporized from a liquid form to produce a corresponding precursor vapor which then is transported to the locus of deposition, to form the material film or coating on the substrate structure. The liquid phase which is vaporized to form the precursor vapor may comprise a liquid-phase source reagent per se, or the source reagent may be dissolved in or mixed with a liquid to facilitate such vaporization to place the source reagent in the vapor phase for the deposition operation.
CVD of carbide materials requires that the carbide source reagents, i.e., the precursor compositions for the carbide material to be deposited, be sufficiently volatile to permit gas phase transport into the chemical vapor deposition reactor. The carbide source reagent must decompose in the CVD reactor to deposit only the desired carbide material at the desired growth temperature(s). Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the transport lines before reaching the reactor deposition chamber. These problems are important in CVD processes that use thermally unstable solid source precursors that display significant decomposition at conditions needed for sublimation. Such decomposition can occur in all reagent delivery systems that involve a vaporization step, not only in the vaporizer in a liquid delivery system but also in more conventional reagent delivery systems that include bubblers and heated vessels operated without carrier gas.
When carbide materials are desired to be deposited, obtaining optimal physical and/or chemical properties requires close control of stoichiometry which can be achieved if the reagent can be delivered into the reactor in a controllable fashion. In addition, the reagents must not be so chemically stable that they do not form the desired coating or film in the deposition chamber. In some cases, the source reagents are solids whose sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it will be very difficult to control the stoichiometry of the deposited films.
CVD and CVI may also be used when it is desired to simply deposit a protective coating on a structural surface, fiber or throughout a porous material or structure. In this circumstance, close attention to stoichiometry and process control are not as important as is the case when the end product must have favorable physical and/or chemical, e.g., conductive, properties. An example of such application would be using CVD or CVI to form a carbide coating for its tribological or bulk strength characteristics. A specific example of such a coating is SiC as applied to ceramic matrix composites or used as a protective coatings on fibers and structures.
One method of delivering reagents or precursors into a CVD/CVI chamber is to directly inject the reagent liquid into the chamber where the reagent decomposes thereby depositing the desired compound on the object surface. When injecting the liquid reagent directly into the reaction chamber, the liquid would likely be sprayed in a mist form by a nebulizer or similar device to more evenly disperse the liquid reagent into the chamber environment. This technique is most appropriate when a protective coating is desired for structural or fiber applications and not for a more controlled, thin coating, as would be the case for a semiconductor material.
An alternative CVD process applicable to the semiconductor industry is liquid delivery, accomplished with the aid of a vaporization zone. In this zone the reagent liquid is flowed onto a flash vaporization matrix structure which is heated to a temperature sufficient to flash vaporize the reagent source liquid. A carrier gas may optionally be flowed by the flash vaporization matrix structure to form a carrier gas mixture containing the flash vaporized reagent source liquid. The use of a flash vaporization matrix structure allows for greater control and better formation of thin, uniform coatings or films as for example is desired in the semiconductor industry.
The means for flowing the precursor liquid onto the flash vaporization matrix or directly into the reaction chamber may comprise any suitable liquid pumping means, such as a positive displacement liquid pump or a liquid mass flow controller.
Within the family of carbides, silicon carbide (SiC) is perhaps the most popular fiber interface coating in oxide and non-oxide reinforced ceramic matrix composites. A common precursor presently used for the vapor deposition of SiC is methyltrichlorosilane (MTCS). The advantages of MTCS include its high volatility and reasonable Si yield, which makes it well suited to CVD/CVI processes. Table 1 shows the relevant physical and chemical properties of MTCS, and includes several which make it a useful precursor for SiC applications.
TABLE 1 ______________________________________ Characteristics of Methyltrichlorosilane (MTCS) ______________________________________ MTCS Formula: CH.sub.3 SiCl.sub.3 Melt. Pt.: -77.8.degree. C. Wt. % Si: 19 Boil. Pt.: 31.degree. C. Molec. Wt.: 149.50 Vapor Pressure: no data Spec. Gravity: 1.27 Flash pt.: 5.degree. C. Thermal Decomposition Products: HCl, CO, SiO.sub.2 Reactivity with Water: Forms toxic, corrosive HCl fumes Hazard Classification: Flammable, Corrosive, Toxic ______________________________________
As a raw material for such applications, MTCS is also relatively inexpensive. There are, however, certain practical disadvantages to using MTCS as a SiC application precursor.
MTCS is highly moisture-sensitive, forming corrosive and toxic HCl fumes upon contact with moisture, a fact that significantly complicates storage, disposal, handling and pumping of such material. Generation of corrosive products in CVD and CVI processes also complicates effluent treatment, both technically and economically. Due to its corrosivity in pumping equipment, delivery of MTCS vapor to the CVD/CVI reactor is best done using bubbler methods. However, bubbler methods have led to poor control in delivery rate of the precursor vapor to the reactor to such extent that customized liquid delivery (pump) systems and/or alternative precursors are desired.
Additional problems associated with the use of MTCS as a SiC precursor include the need to use very large amounts of hydrogen to transport the MTCS vapor and provide the reducing atmosphere needed to form SiC. This high volume use of hydrogen poses both hazard risks and cost penalties.
A further disadvantage of using MTCS for deposition of SiC is that, under typical operating conditions, the required temperature range falls approximately between 1000-1400.degree. C. Unfortunately, most available substrates for which SiC and other similar coatings are desired, have a temperature limit of between 800-1200.degree. C. Lastly, MTCS can also yield detrimental oxygen incorporation in SiC coatings formed using such source reagent.
Accordingly, it is an object of the present invention to provide a composition and method for the formation of carbide materials on substrates.
It is another object of the invention to provide an improved process for CVD/CVI of silicon carbide.
It is a still further object of the present invention to provide a method for forming carbide coatings on fibers, surfaces and other substrates using the CVD process, that is safer than the CVD carbide coating forming processes of the prior art.
It is yet another object of the invention to provide a CVD process for forming carbides on a substrate, which is able to be carried out at lower temperatures than processes of the prior art for forming SiC coatings.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.